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Chain Line Problem(3 posts)

Chain Line Problemkg1
Sep 29, 2002 10:52 AM
I got a new Gunnar Roadie this year with Ultegra from a well-respected shop. When I picked it up, I couldn't use the four smallest rear cogs (12-25 cassette, double) if I was in the small chain ring without having the chain rub on the big ring (very annoying). Thanks to the advise I received here, I brought the bike back and told the LBS that's not the way things are supposed to work, please fix it. When I picked the bike up about one week later, I could use all but the smallest two cogs without chain rub. Good enough for me.

On the way home on Friday, the chain started to skip like crazy. I stopped in at an LBS on the route home and the mechanic (a very helpful guy who wanted to charge me $2.10 for about a half-hour's work) told me that my chain line was terrible, resulting in a badly gouged big ring, and a chain that was shot after about 1200 miles. He also took off the cranks and said that I had a 118.5 mm bottom bracket and that I should only have a 109.5 mm bb.

I want to bring the bike back to the shop I got it from and ask them to fix the chain line. Any advice you can give me on the causes of a chain line problem will make me a more informed consumer, and will be much appreciated.

Thanks.
re: Chain Line ProblemRusty Coggs
Sep 29, 2002 12:31 PM
The 118.5 BB is for a triple crankset and the 109.5 is for a double.
Something's really wrong here!Kerry
Sep 29, 2002 4:32 PM
You should be able to pretty much see your chain line quality - if you sight down a line between the chain rings, it should hit dead center on the cassette. The fact that you had chain rub in the 4 smallest cogs suggests that the chain rings were too far to the left when viewed from the back of the bike. It sounds like your original shop fixed this by installing a triple BB. However, this should have moved the crank WAY to the right and eliminated any chain rub, while perhaps giving you problems when on the big ring and the larger cogs in the back. If you really do have the triple BB, then going to a double BB would just take you back to your original problem. All of this (if the facts as stated are true) points to something seriously goofy with the frame.

Start by determining your chain line. Take a long straight edge (e.g. carpenter's square) and check the chain line. If the chain line is correct now, then you're getting bad advice from mechanic #2. If the chain line is off (chain rings too far to the right) then it's time to put that frame on an alignment table and find out what's up.